1. Field of the Invention
This invention relates to an optical element for use in a laser beam printer or a display and to an optical apparatus to which such element is applied.
2. Description of the Prior Art
As regards a light beam used in a laser beam printer or the like, it is generally known that an intensity distribution having an intensity of the marginal portion equal to or higher than the intensity of the central portion may result in more emphasized edge portion of the marginal portion of the spot of the light beam which may in turn result in higher resolution of printed-out matter than an intensity distribution having an intensity of the central portion higher than the intensity of the marginal portion, such as the Gaussian type intensity distribution of a laser beam which is a light source. A typical optical element used in such a laser beam printer or a light modulating element used therein for changing the unsuitable intensity distribution of the incident light beam into a suitable intensity distribution comprises a combination of two conical lenses, but it is difficult and impractical to work this element so as to have no eccentricity or to arrange and adjust this element to provide the angle of inclination of the surface of the element with respect to the optic axis within a set range. The optical element for changing the intensity distribution of such incident light beam is also required as an illuminating optical system for a light shutter array or the like. That is, where a laser beam is used at its intensity distribution as an illuminating light source, the intensity of the light of the marginal portion of the illuminating light beam is lower than that of the central portion of the illuminating light beam and this causes irregularity of the quantity of illuminating light to be created in the member to be illuminated. To eliminate this, there is known a method of enlarging the beam diameter of the laser beam sufficiently greatly as compared with the member to be illuminated, and illuminating said member by the use of a region which can be regarded as being practically uniform, but in this method, considerable part of the energy of the illuminating light which does not illuminate the member to be illuminated is wasted. Also, the use of the conical lenses as described above would cause a similar inconvenience.
In U.S. Pat. No. 4,370,026, it is disclosed that light beams divided into two by a prism are combined together by two mirrors to thereby achieve the uniformization of the distribution of quantity of light in the cross-section of a laser beam. However, it encounters a difficulty of alignment in the optical arrangement to discretely dispose the optical element for separating the laser beam and the optical element for re-combining the separated laser beams, as is disclosed in said U.S. patent.